A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A key factor limiting pattern printing is the wavelength λ of the radiation used. In order to be able to project ever smaller structures onto substrates, it has been proposed to use extreme ultraviolet (EUV) radiation which is electromagnetic radiation having a wavelength within the range of 10-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such EUV radiation is sometimes termed soft x-ray. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings.
EUV sources based on a tin (Sn) plasma do not only emit the desired in-band EUV radiation but also out-of-band radiation, most notably in the deep UV (DUV) range (100-400 nm). Moreover, in the case of Laser Produced Plasma (LPP) EUV sources, the infrared radiation from the laser, usually at 10.6 μm, presents a significant amount of unwanted radiation. Since the optics of the EUV lithographic system generally have substantial reflectivity at these wavelengths, the unwanted radiation propagates into the lithography tool with significant power if no measures are taken.
In a lithographic apparatus, out-of-band radiation should be minimized for several reasons. Firstly, resist is sensitive to out-of-band wavelengths, and thus the image quality may be deteriorated. Secondly, unwanted radiation, especially the 10.6 μm radiation in LPP sources, leads to unwanted heating of the mask, wafer and optics. In order to bring unwanted radiation within specified limits, spectral purity filters (SPFs) are being developed.
Spectral purity filters can be either reflective or transmissive for EUV radiation. Implementation of a reflective SPF requires modification of an existing minor or insertion of an additional reflective element. A transmissive SPF is typically placed between the collector and the illuminator and, in principle at least, does not affect the radiation path. This may be an advantage because it results in flexibility and compatibility with other SPFs.
Grid SPFs form a class of transmissive SPFs that may be used when the unwanted radiation has a much larger wavelength than the EUV radiation, for example in the case of 10.6 μm radiation in LPP sources. Grid SPFs contain apertures with a size of the order of the wavelength to be suppressed. The suppression mechanism may vary among different types of grid SPFs as described in the prior art and detailed embodiments further in this document. Since the wavelength of EUV radiation (13.5 nm) is much smaller than the size of the apertures (typically >3 μm), EUV radiation is transmitted through the apertures without substantial diffraction.
Several prior art spectral purity filters (SPFs) rely on a grid with micron-sized apertures to suppress unwanted radiation. U.S. Patent Application Publication 2006/0146413 discloses a spectral purity filter (SPF) comprising an array of apertures with diameters up to 20 μm.
Depending on the size of the apertures compared to the radiation wavelength, the SPF may suppress unwanted radiation by different mechanisms. If the aperture size is smaller than approximately half of the (unwanted) wavelength, the SPF reflects virtually all radiation of this wavelength. If the aperture size is larger, but still of the order of the wavelength, the radiation is at least partially diffracted and may be absorbed in a waveguide inside the aperture.
The approximate material parameters and specifications for these SPFs are known. However, manufacturing is not straightforward at these specifications. The most challenging specifications are: apertures of typically 4 μm in diameter; a grid thickness of typically 5-10 μm; very thin (typically <1 μm) and parallel (non-tapered) walls between the apertures to ensure maximal EUV transmission.
Silicon has emerged as a promising material for the manufacture of such grids, using the photolithographic patterning and anisotropic etching processes that are well understood from semiconductor manufacturing. For deep apertures with a well-controlled cross-section, deep reactive ion etching (DRIE) has been found promising, although difficulties remain in providing a method of manufacturing an EUV spectral purity filter with the desired specifications.